OTOH, I find the idea behind the Tarrasque very instructive, although deadly. ... Maybe there should be earlier, less deadly monsters on the same theme? Or there should be a warning level for spell frequency in the monster recall, like "casts spells very frequently" in red?

There is at least one: Farmer Maggot. If you get him while you're still wimpy he takes FOREVER to kill.

There is at least one: Farmer Maggot. If you get him while you're still wimpy he takes FOREVER to kill.

Yup...I chased him around the store forever once with a dagger. Finally realized he wasn't ever going to fall...stepped into the store and bought a main gauche....went back after him and he fell a few turns later.

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“We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see.”
― Tom Stoppard, Rosencrantz and Guildenstern are Dead